The present invention relates to a linear variable differential transformer (shortly xe2x80x98LVDTxe2x80x99 in the following) widely used for measurement of displacements and particularly to a high sensitivity displacement measuring device using the LVDT, which can handle submicron measurements with a high indifference to surrounding environment, noise and the like and with an increased resolution.
The sensitivity used in the field of the displacement measurements represents the ratio of the analog output voltage relative to the input displacement to be measured in a displacement measuring device using a LVDT. For example, a high sensitivity means that the output voltage is largely generated with respect to a predetermined displacement
For such an ordinary analog output voltage generator, the resolution is closely related to the noise and sensitivity.
Resolution=Noise/Sensitivity
In practical circumstances, the resolution is influenced severely by the noises caused by the surrounding environment or various other factors. A high sensitivity is advantageous in order that the insensibility to outside factors can be maintained and the high resolution can be high enough to realize application in submicron area, with the minimum measurable displacement lowered
FIG. 1 shows schematically a displacement measuring device using a LVDT according to the conventional art.
As shown in FIG. 1, the displacement measuring device according to the conventional art has the form of cylinder, in which there is disposed a primary coil bundle 2 in cylindrical form, with the secondary coil bundles 3H and 3L positioned on the top and bottom thereof. In the center of the primary coil bundle 2 and secondary coil bundles 3H and 3L, there is disposed a magnetic core 4, to the lower end of which there is connected a rod 5, having a contact probe 6 disposed at its lower end. Under the lower secondary coil bundle 3L, ball bearings 8 are disposed around the rod 5 to facilitate the vertical movement of the rod 5, with the movement of which there expands or contracts a spring 9 disposed below the ball bearings 8. The primary coil bundle 2, upper and lower coil bundles 3H and 3L, ball bearings 8 and spring 9 are housed in a housing 7, under which housing the lower part of contact probe 6 protrudes.
Thus the contact probe 6 protruding from the bottom end of the housing 7 moves up or down according as the device detects a displacement, thereby the magnetic core 4 connected to the rod 5 moves up or down the same distance.
On the other hand, when an electric voltage is applied to the primary coil bundle 2, a magnetic field is generated within the displacement measuring device 1, causing the magnetic core 4 to move in accordance with the displacement input. Thus, the distribution of magnetic fields in the respective secondary coil bundles 3H and 3L within the displacement measuring device 1 are changed and so the values for the difference in voltage in the form of differential voltages which are induced in the respective secondary coil bundles 3H and 3L due to the change in the magnetic fields are also changed, wherein the values for such differential voltages are proportional to the displacement inputs.
For such displacement measuring devices based on the conventional LVDT, an improvement in the sensitivity can be expected when the ratio of the windings for the secondary coil bundles to the windings for the primary coil bundle are large. Therefore, the number of the windings for the secondary coil bundles needs to be increased to increase the measurement sensitivity, with the result that the size of an overall displacement measuring device must be increased.
However, if the coil windings are more increased, the number of the coil windings must be limited due to capacity saturations, generation of non-linear elements or the like. Therefore, there was a disadvantage that a high selectivity is hardly attained
In addition, the type of measuring device with the components of a guide such as ball bearings, springs and the like has the drawback that sub-micron resolving power can not be attained because of the intrinsic non-linearity.
The present invention was created to resolve the above-described problems with the conventional art and the object of the present invention is to provide a super-precision high sensitivity displacement measuring device which is improved in its construction to facilitate a super-precision measurement and which has such a high resolving power as to be able to make sub-micron measurement due to increased sensitivity by using a guiding means for amplifying an input displacement
To that end, there is provided according to the invention a displacement measuring device with position resolving power, comprising: an electromagnetic system which forms a closed loop of magnetic bodies and which houses primary coils and secondary coils for forming magnetic field within said closed loop; guiding means which include displacement input parts and displacement output regions and which act to guide so that the displacement output regions can produce an output amplified in proportional to the displacement input to the displacement input parts, and a supporting means for supporting the displacement input parts of said guiding means so that the displacement may be input only in one axial direction.
According to another aspect of the invention, there is provided a displacement measuring device, wherein said electromagnetic system includes a plurality of loop magnets in E-form, beam magnets connecting the upper and lower free ends of said loop magnets to form a closed loop, a plurality of primary coils wound around inward projections of said loop magnets, a plurality of magnetic cores which extend parallel to said beam magnets between opposite primary coils and which are positioned at a predetermined distance of less than millimeters from the end of inward projection of a loop magnet, the inward projection being wound by primary coil, and a plurality of secondary coils winding around said magnetic cores, wherein the magnetic cores are fixed, at their ends, to the displacement output regions of said guiding means.
According to still other aspect of the invention, there is provided a displacement measuring device, wherein said cores and said beam magnets are of the same material.
According to still other aspect of the invention, there is provided a displacement measuring device, wherein said guiding means, as plate springs with a thickness of some hundred micrometers, are so constructed that, below the lower edges of said plate springs there protrude the displacement input parts, above the displacement input parts there are positioned fixing regions, attached to the beam magnets, and on both sides of the fixing regions there are connected rotatable or tiltable regions, and that said fixing regions and said rotatable regions are provided in a symmetric manner in the upper edges as well beside the lower edges of the plate springs and displacement output regions connect the rotatable regions on the upper and lower edges of the plate spring; and wherein between a fixing region and rotatable regions, between a displacement input part and rotatable legions and between rotatable regions and a displacement output region, a connecting section or sections are connected, so that when a displacement is input in the displacement input parts, rotatable regions rotate around the connecting sections, as fulcrums, connecting the fixing legions and the rotatable regions to cause the displacement output as determined by a mathematical equation at the displacement output regions.
According to an aspect of the invention, there is provided a displacement measuring device, wherein said plate springs are made of beryllium-copper and are attached to both sides of said electromagnetic system so that the both plate springs may extend parallel to the closed loop of the electromagnetic system.
According to an aspect of the invention, there is provided a displacement measuring device, wherein said supporting means comprises a fixing block to be attached to the electromagnetic system, a movable block to have a contact probe mounted and to be fixed to the displacement input parts of said guiding means and supporting beams to connect said fixing block with said movable block.
According to still other aspect of the invention, there is provided a displacement measuring device, wherein said movable block is fixed, on its both faces, to the displacement input parts of the two sheets of plate springs on the both sides of said electromagnetic system, and said contact probe mounted on the movable block is oriented in the direction opposite to the beam magnets.
And, according to still other aspect of the invention, there is provided a displacement measuring device, wherein said supporting beams have the form of flat bars and connect the upper and lower faces of both the fixing block fixed to the bottom surface of the electromagnetic system and the movable block.
Thus, the high sensitivity displacement measuring device according to the present invention has the advantage that measurements in the sub-micron area can be easily made through the improvement in, for example, the coil arrangement, the closed loop structure and the decreased gaps at the opposite ends of magnetic cores, and particularly through the improvement in the output sensitivity based on the increased displacement amplification of the magnetic cores by the help of the displacement amplification mechanism of leaf springs, and that the control of the contact force for the probe is possible through the adjustment of the elastic modulus for leaf springs and the distances between connecting sections on the leaf springs.
Further, the present invention has another advantage that the movable block moves only in one direction, with supporting bars bearing the block, and the structure is less sensitive to the variation in external environment, i.e. noise or changing temperature thanks to the same material for both the beam magnets and the magnetic cores.